idm_payloads.py

"""
Helpers for building iDotMatrix image payloads.

These are intentionally kept compatible with idotmatrix.modules.image.Image._createPayloads
so they can be swapped out later if the library exposes a public API.
"""

from typing import List
import struct


def split_into_chunks(data: bytes, chunk_size: int) -> List[bytes]:
    return [data[i : i + chunk_size] for i in range(0, len(data), chunk_size)]


def create_payloads(png_data: bytes) -> bytearray:
    """
    Wrap PNG bytes into the device's custom payload framing.
    """
    png_chunks = split_into_chunks(png_data, 4096)

    # Matches the library's behavior: len(data) + number of chunks
    idk = len(png_data) + len(png_chunks)
    idk_bytes = struct.pack("h", idk)  # 16‑bit signed int
    png_len_bytes = struct.pack("i", len(png_data))  # 32‑bit signed int

    payloads = bytearray()
    for i, chunk in enumerate(png_chunks):
        flag = 2 if i > 0 else 0
        payload = idk_bytes + bytearray([0, 0, flag]) + png_len_bytes + chunk
        payloads.extend(payload)

    return payloads